1. Field of the Invention
The present invention relates generally to a speed-torque control for an electric motor and in particular to a system for controlling a constant field, direct current motor to define a speed-torque curve to limit the tension applied to a web material being wound on a surface wind machine.
2. Description of the Prior Art
Over the years, the demand for plastic, cloth, paper and metal materials in sheet form has increased so that winding system having improved capabilities have been required. One of the most important parameters in the winding process is the tension applied to the material as it is wound on the take-up roller. The tension determines the tightness and compression in the roll of wound material; the probability of breakage of the material with loss of material and time; and, in some cases, the thickness of the material. Therefore, it is desirable to be able to control the tension applied to the material so as to produce a high quality product.
In some operations it is desirable to maintain a constant tension, while, in others, it is sufficient to define a speed-torque curve which limits the maximum torque which can be applied by the motor. Most control systems utilize some form of direct current motors which are more easily controlled than other types of motors in terms of speed and/or torque output. Since the diameter of the roller increases as the material is being wound, the motor torque must be increased to compensate for the increased moment arm from the center of the roll drive shaft to the periphery of the roll if constant tension is to be applied to the material. One early method of control was a roll diameter measuring rheostat which controlled the field excitation of the motor. Such a mechanical connection to the roll was susceptible to damage when there was a breakage in the material. In the alternative, the motor was supplied with power from a main generator. A tensiometer riding on the web generated an electrical signal representing tension to an amplidyne generator which in turn adjusted the field excitation for the main generator to control the torque generated by the motor. The prior art also employed a dancer arm for sensing web tension. The arm was pivotally mounted and the web was passed around a dancer roller wherein tension on the web tended to rotate the dancer arm which was biased, for example, by a pneumatic cylinder connected to the arm. The position of the arm was sensed to control the speed of the winder motor which was applying the tension to the web. As the diameter of the roll increased, the speed of the motor was decreased so that more torque was applied to the roll to equal the product of the constant tension and the increasing roll diameter.
However, some winding operations do not require constant tension and can produce an acceptable product by controlling the motor at constant speed, at constant torque or along a limited speed-torque curve. In a constant field direct current motor, such as a permanent magnet field, wound rotor motor or a shunt motor having a separately excited field, the armature voltage is proportional to the motor speed and the armature current is proportional to the motor torque. Therefore, the motor control systems can sense either or both of the armature voltage and current to compare with one or more control signals for defining the speed-torque point at which the motor operates for a given load. For example, one prior art control system regulates the speed of a shunt field, direct current motor so as to maintain constant speed over a wide torque range. A first feedback signal is generated as a function of the counter--E.M.F. during free-wheeling and a second feedback signal is generated as a function of the armature current during free-wheeling. These two feedback signals are then combined in such a manner that the energy supplied to the armature is decreased with increasing counter--E.M.F. and is increased with increasing armature current so as to provide substantially constant speed. Such a control system is disclosed in U.S. Pat. No. 3,470,437.
Another form of prior art control circuit is one utilizing armature voltage and current feedback to maintain constant speed up to a predetermined torque and constant torque thereafter. The circuit compares a signal proportional to the motor speed with a desired speed signal to maintain constant speed. When a predetermined armature current is reached, the circuit bypasses the speed control and a signal proportional to the motor torque is compared with a desired torque signal to maintain constant torque. U.S. Pat. No. 3,475,672 discloses a motor control system which functions in this manner.
A further form of prior art control circuit simultaneously utilizes speed and torque proportional signals to define a speed-torque curve for a motor. A signal representing the motor current is compared to a motor-current reference signal to control the speed of the motor. The current reference signal is changed in dependence upon signals corresponding to the relationship between the actual speed of the motor and the motor speed corresponding to reel initial winding speed at the existing web speed to achieve the desired torque variation under loaded conditions and to limit sharply the speed of the motor under unloaded conditions. The signal for changing the current reference potential is derived by comparing the potential across the motor with a motor reference potential. When the motor reference potential is higher than the potential across the motor corresponding to a loaded condition, a given proportion of the difference is added to the motor current reference potential. This, in turn, demands that the current drawn by the motor increase correspondingly, and this increase is effected by controlling the power supply unit for the motor. When the motor is unloaded and is free-wheeling, the potential across the motor becomes greater than the motor-potential reference, and under these circumstances, a substantially higher proportion of the difference between the motor potential and the motor potential reference potential is subtracted from the motor current reference potential so that the power supply unit is actuated to supply substantially less power to the motor and the speed of the motor is limited. Thus, under loaded conditions, the torque of the motor and its speed follows a predetermined desired pattern and under unloaded conditions, the speed of the motor is limited. Such a control system is disclosed in U.S. Pat. No. 3,073,996.